1. Field of the Invention
The present invention relates to an intermediate transfer belt, and to an image forming apparatus using the intermediate transfer belt.
2. Description of the Related Art
In the conventional art, a belt, especially a seamless belt, has been used for various purpose, as a member in an electrophotographic image forming apparatus. In recent years, an intermediate transfer belt system has been used in a full color image forming apparatus, where the intermediate transfer belt system includes superimposing developed images of four colors, yellow, magenta, cyan, and black temporarily on an intermediate transfer member, and collectively transferring the superimposed images onto a transfer medium, such as paper.
As for the aforementioned intermediate transfer belt, a system using developing units of four respective colors to one photoconductor has been used, but this system has a problem that a printing speed thereof is slow. Accordingly, to achieve high speed printing, a quarto-tandem system has been used, where the tandem system includes providing photoconductors of four respective colors, and an image of each color is continuously transferred to paper.
In this system, however, it is very difficult to accurately position images of colors to be superimposed, as the paper is affected by the fluctuations of the environment, which causing displacement of the colors in the image.
Accordingly, currently, an intermediate transfer belt system has been mainly adapted for the quarto-tandem system.
Under the circumstances as mentioned above, the higher requirements for properties (high speed transferring, and accuracy for positioning) of a intermediate transfer belt have been demanded than before, and therefore it is necessary for an intermediate transfer belt to satisfy these requirements. Especially for the accuracy for positioning, it has been required to inhibit variations caused by deformation of an intermediate transfer belt itself, such as stretching, after continuous use thereof. Moreover, an intermediate transfer belt is desired to have flame resistance as it is provided over a wide region of a device, and high voltage is applied thereto for transferring. To satisfy these demands, a polyimide resin, polyamideimide resin, or the like, that is a highly elastic and highly heat resistant resin, has been mainly used as a material of an intermediate transfer belt.
An intermediate transfer belt formed of a polyimide resin is however has high surface hardness, as the resin constituting the belt has high hardness. When a toner image is transferred, therefore, high pressure is applied to a toner layer. As a result, the toner is partially aggregated, and formation of so-called a partially-missing image, where part of an image is not transferred, may be caused. Moreover, the intermediate transfer belt having high hardness has inferior correspondence to a member to be in contact in a transferring section, such as with a photoconductor or a sheet. Accordingly, a contact failed area (void) may be partially caused in the transferring section, causing transfer unevenness.
In addition, there have been recently more occasions that images are formed on various types of a sheet in a full-color electrophotographic system. Not only a general smooth sheet, various sheets from a sheet of high smoothness with slippy surface, such as coated paper to a sheet having surface roughness, such as recycle paper, embossed paper, Japanese paper, and Kraft paper, have been often used. It is important that an intermediate transfer belt has correspondence to sheets of these different surface configurations. If the intermediate transfer belt has insufficient correspondence, unevenness in density and color tone corresponded to the convexoconcave surface configurations of the sheet appears in the resulting image. To solve this problem, various intermediate transfer belts have been proposed to laminate a relatively flexible layer onto a base layer.
When the relatively flexible layer (elastic layer) is used as a surface layer, however, transfer pressure reduces, and correspondence to surface irregularities of a sheet improves, but a toner may not be desirably released from an intermediate transfer belt because of insufficient releasing properties of a surface of the belt, which reduces transfer efficiency, and cannot effectively utilize the aforementioned effects. Moreover, such intermediate transfer belt also has problems that it has low abrasion resistance and excoriation resistance.
The aforementioned elastic layer is often designed to be the thicker film than a base layer to give an effect of providing correspondence to surface irregularities. A flexible material called an elastic layer generally has greater heat expansion coefficient than that of a base layer formed of polyimide or the like. Therefore, the resulting belt significantly curls outwards. When such belt is mounted in an image forming apparatus, running failures and cleaning failures tend to be caused.
To solve the aforementioned problem, there is disclosed an intermediate transfer belt, in which materials each having different heat expansion coefficient are laminated, a member for inhibiting curling is incorporated in the edge of the belt curled outwards (see Japanese Patent Application Laid Open (JP-A) No. 2005-309181). The disclosed belt however tends to be abraded by contact with the inhibiting member, and moreover the transfer pressure tends to varied between a center part and edge part of the intermediate transfer belt to thereby cause variations in a transfer performance. Therefore, the belt cannot provide images of high quality level of which is demanded for the current image forming apparatus.
There has been also disclosed a semiconductor belt containing an outer layer and inner layer both of which are mainly formed of a polyimide resin, where a difference in linear expansion coefficient between the outer layer and the inner layer is 30 ppm/° C. or lower (see Japanese Patent (JP-B) No. 4396959). The disclosed belt exhibits an effect of reducing a curling amount when the thickness thereof is approximately up to 200 μm, but it is difficult to control the curling amount when the thickness thereof is 200 μm or greater.
Particularly in the recent trends, a size of a device has been increased corresponded to the increase in the speed of printing, e.g., sheets per unit time, of an image forming apparatus. Accordingly, as for an intermediate transfer belt, a belt having a long peripheral length is desired.
Such intermediate transfer belt has a large degree of thermal shrinkage compared to the conventional intermediate transfer belt, and therefore edge parts of the intermediate transfer belt are more easily curled. As a result, there is a problem that the edge parts of the belt are easily damaged.
Accordingly, it is currently desired to provide an intermediate transfer belt capable of forming high quality images, having excellent durability without causing paper jamming due to the curl of the belt and forming uneven images, and having excellent cleaning properties.